1. Field of the Invention
The invention relates to an optical pickup for performing optical information writing or information reading on or from such an information recording medium as a compact disc (CD) or a digital versatile disc (DVD).
The present application claims each priority from Japanese Patent Application Nos. 2001-315012 and 2002-280259, the disclosure of which is incorporated herein by reference.
2. Description of the Related Art
The development of large-capacity information recording media such as CDs and DVDs, or that of write-once type or rewritable type CDs and DVDs in particular, has resulted in improved functionality.
FIG. 1 shows the configuration of an optical pickup which has conventionally been used for writing, reading or erasing information on an information recording medium (hereinafter referred to as “disc DSC”) such as these CDs or DVDs.
In FIG. 1, when writing information on write-once type or rewritable type disc DSC, S-polarized laser light B1 which is modulated with the information, such as sound data and image data, to be recorded is emitted from a semiconductor laser diode LD. The laser light B1 is reflected/transmitted from/through a half mirror HM.
Reflected light B2 reflected from the half mirror HM passes through a quarter-wavelength plate WLP and an objective lens OBL to create spot write light Bw. The write light Bw is irradiated on the disc DSC for writing information.
Reflected light Bwr, or the reflection resulting from the irradiation onto the disc DSC with the write light Bw, passes through the objective lens OBL and the quarter-wavelength plate WLP. Of the reflected light Bwr incident upon the half mirror HM, light Bwd transmitted through the half mirror HM is converted into a photoelectric conversion signal by an optoelectronic integrated circuit OEIC. Based on a various kind of error signal included in the photoelectric conversion signal, a various kind of control such as a focus servo is conducted in order to adjust the objective lens OBL to an appropriate position.
A part of the laser light B1 is transmitted through the half mirror HM to create partial transmitted light B3, which is detected by a photodiode PD. Based on the detection signal thereof, the semiconductor laser diode LD is feedback-controlled in emission power so that the power of the write light Bw is maintained at an appropriate value while writing information, and also that of the light Bwd received by the optoelectronic integrated circuit OEIC is maintained at an appropriate value to improve an accuracy of the focus servo or the like.
In other words, it is preferable to set the power of the write light Bw at the appropriate value in order to improve the accuracy of the information writing by recording an appropriate record mark or the like. Further, in order to improve the accuracy of the focus servo or the like, it is preferable to maintain the power of the light Bwd received by the optoelectronic integrated circuit OEIC at the appropriate value. For doing so, the semiconductor laser diode LD is feedback-controlled in the emission power based on the detection signal of the photodiode PD, thereby maintaining the both power of the write light Bw and the light Bwd received by the optoelectronic integrated circuit OEIC at the respective appropriate values.
In reading information to reproduce the information already recorded on a read-only type, write-once type, or a rewritable type disc DSC, the S-polarized laser light B1 of certain power is emitted from the semiconductor laser diode LD. The reflected light B2 reflected at the half mirror HM passes through the quarter-wavelength plate WLP and the objective lens OBL to create spot read light Br, with which the disc DSC is irradiated.
Then, the read light Br is reflected from the disc DSC to cause return light Brr which carries the characteristics of the information recorded on the disc DSC. Of the return light Brr returning to the half mirror HM through the objective lens OBL and the quarter-wavelength plate WLP, light Brd transmitted through the half mirror HM is received by the optoelectronic integrated circuit OEIC. As a result, a photoelectric conversion signal (RF signal or the like) carrying the characteristics of the information recorded on the disc DSC is created and supplied to a signal processing circuit such as a decoder circuit for the sake of information reproduction.
Additionally, a various kind of control such as a focus servo and a tracking servo is conducted based on a various kind of error signal included in the photoelectric conversion signal being output from the optoelectronic integrated circuit OEIC to adjust the objective lens OBL at an appropriate position.
In information reading, as with information writing, part of the laser light B1 is transmitted through the half mirror HM to create the partial transmitted light B3, which is detected by the photodiode PD. Based on the detection signal thereof, the semiconductor laser diode LD is feedback-controlled in emission power so that the power of the read light Br and that of the light Brd received by the optoelectronic integrated circuit OEIC are maintained at respective appropriate values during information reading.
In information erasing to erase information already recorded on the rewritable type disc DSC, a power of S-polarized laser light B1 is set so as to erase a record mark recorded on the disc DSC, and the laser light B1 is emitted from the semiconductor laser diode LD. Erase light Be is irradiated onto the disc DSC after passing through the half mirror HM, the quarter-wavelength plate WLP, and then the objective lens OBL as with the above mentioned information writing. Thus, in such a case as a rewritable type disc DSC comprising a record layer formed with, for example, phase change material, a crystalloid of the record mark already recorded on the record layer is changed by the irradiated power to erase information.
Further, in this information erasing also, reflected light Ber reflected from the disc DSC is incident on the optoelectronic integrated circuit OEIC through the objective lens OBL, the quarter-wavelength plate WLP and the half mirror HM to cause photoelectric conversion of light Bed transmitted through the half mirror HM. Based on a various kind of error signal included in the photoelectric conversion signal, a various kind of control such as the focus servo and the tracking servo is conducted in order to adjust the objective lens OBL to an appropriate position.
The part of the laser light B1 is transmitted through the half mirror HM to create the partial transmitted light B3, which is detected by the photodiode PD. Based on the detection signal thereof, the semiconductor laser diode LD is feedback-controlled in the emission power so that the power of the erase light Be and that of the light Bed received by the optoelectronic integrated circuit OEIC are maintained at respective appropriate values during information erasing.
Now, at the occasion of information writing, information erasing or information reading as described above, part of the reflected light Bwr, Ber or the return light Brr may be reflected from the half mirror HM, causing the phenomenon that partial reflected light Hwr, Her or Hrr thereof returns to the emission end of the semiconductor laser diode LD. The quarter-wavelength plate WLP is thus arranged to avoid the adverse effect of the phenomenon.
More specifically, suppose that there were arranged no quarter-wavelength plate WLP. When the S-polarized laser light B1 is emitted from the semiconductor laser diode LD, the reflected light Bwr, Ber or return light Brr form the disc DSC would reach the half mirror HM just as the S-polarized laser light B1 would do so. Besides, the partial reflected light Hwr, Her or Hrr reflected from the half mirror HM would be incident on the emission end of the semiconductor laser diode LD just as is S-polarized. If the partial reflected light Hwr, Her or Hrr S-polarized in the same fashion as the laser light B1 is incident on the emission end of the semiconductor laser diode LD, there would occur such problems that the semiconductor laser diode LD varies in the emission power and that it generates noise-containing laser light B1.
In contrast, given that the quarter-wavelength plate WLP is arranged, the reflected light B2 reflected from the half mirror HM is converted from the S-polarization to a circular polarization when passing through the quarter-wavelength plate WLP. The disc DSC is thus irradiated with the circularly-polarized write light Bw, erase light Be or read light Br. The reflected light Bwr, Ber or return light Brr reflected back from the disc DSC is incident on the quarter-wavelength plate WLP as is circularly polarized. The incident light is converted from the circular polarization to a P-polarization when passing through the quarter-wavelength plate WLP, and reaches the half mirror HM.
Note that in the foregoing conversion from the circular polarization to the P-polarization, the reflected light Bwr, Ber or return light Brr reaching the half mirror HM is P-polarized in a direction 90° different from the direction of polarization of the laser light B1. The partial reflected light Hwr, Her or Hrr reflected from the half mirror HM is thus incident on the emission end of the semiconductor laser diode LD as is P-polarized with 90° difference from the S-polarization of the laser light B1.
Consequently, from the provision of the quarter-wavelength plate WLP, it follows that the partial reflected light Hwr, Her or Hrr P-polarized with the 90° difference from the laser light B1 having the S-polarization is incident on the emission end of the semiconductor laser diode LD. The adverse effect on the semiconductor laser diode LD can thus be reduced to generate the laser light B1 which is low in noise or the like.
As mentioned above, the conventional optical pickup is provided with the quarter-wavelength plate WLP so that the semiconductor laser diode LD operates normally, and further feedback-controls the emission power of the semiconductor laser diode LD based on the detection output from the photodiode Pd so that the disc DSC is irradiated with the write light Bw, erase light Be, or read light Br having an appropriate power, and the light Bwd, Bed, or Brd with each power appropriately maintained is incident on the optoelectronic integrated circuit OEIC.
However, when the laser light Bi may vary in wavelength depending on the potential properties of the semiconductor laser diode LD, such as changes in ambient temperature and internal temperature (hereinafter, referred to as “changes in environmental temperature”), there occurred the following problems to be solved.
That is, when the wavelength of the laser light B1 varies, the properties of the optical elements constituting the optical pickup substantially vary according to the difference in the wavelength of the laser light B1.
In detail, there occurs a problem that each power of the write light Bw, erase light Be, and read light Br is off the appropriate value due to the variation of the wavelength. In addition, each power of the above mentioned light Bwd, Bed, and Brd incident on the optoelectronic integrated circuit OEIC is also off the respective appropriate values so that the accuracy in such controls as the focus servo and the tracking servo is lowered, and also quality of reproduced signals obtained in the information reproduction is lowered.
More specifically, the following problem occurs if the wavelength of the laser light B1 varies to cause variations in the transmittance and the reflectance of the half mirror HM.
That is, suppose that the half mirror HM has a transmittance of Ts when the laser light B1 with the S-polarization which is emitted from the semiconductor laser diode LD is incident thereon, and a transmittance of Tp when the reflected light Bwr, Ber or return light Brr from the quarter-wavelength plate WLP is incident thereon.
Then, when the laser light B1 varies in wavelength depending on such factors as changes in environmental temperature, the half mirror HM varies in those transmittances Ts and Tp according to dependency from the wavelength, thereby causing each power of the light Bwd, Bed or Brd incident on the optoelectronic integrated circuit OEIC to be off the respective appropriate values. In other words, the accuracy of various controls such as the focus servo and the tracking servo is lowered.